I don't know if it becomes "depleted"... beneficial bacteria live on the surface of the sand, and they don't come off when the sand is cleaned by vacuuming. The volume of sand itself may deplete a bit as a bit is lost when vacuuming. What we do is just replenish the sand from time to time by adding more Arag-Alive when we feel it's a bit thin.
Using this method to maintain the sandbed, the only time we really discard and replace a sand bed is if we're taking down and moving a tank. The main reason why we do this, is so we can empty the tank completely to move it, and even with regular cleaning there are bound to be spots that are hard to access, and some junk will accumulate there - no sense keeping that junk. Also, once one drains a tank and there's no water movement, those bacteria rapidly begin to die off. Just ask anybody who has kept a bucket of used sand for more than a few hours - once you stir it up, it stinks. Decaying organic matter, dead bacteria... stinky mess. Easier and safer to just replace it at that point, and start fresh.
Of course if we are hired to take over maintenance on a tank where proper sand bed maintenance hasn't been done, we will remove some sand if we find it necessary. I've pulled a few smelly plenums over the years too (that's a whole other can of worms). In those instances we usually pull all the livestock first, and before we start we make the customer aware of the risks we've discussed - that disturbing things during the removal can also be a risk factor. So far we've been lucky though (knock on wood!)
Been doing it this way for over 10 years, have not had an issue yet. Don't forget that those beneficial bacterial also inhabit the rock, so it's not like it's a huge wipe-out of biological. Yes, removing the existing sand bed will remove a lot of beneficial bacteria but that's the safer option versus replacing the same sand bed full of a pile of dead bacteria.
Jenn
